'....Therefore messages delivered through the POP3 connector won’t be scanned by IMF'

Is that still true / is there a work around? We have an outside web / email hosting service and figured using the pop3 connector would be an easy way to have shared contacts / calendars but keep things simple. I was hoping to be able to use IMF, but I guess if the mail is hosted off site, I can / should look into using that hosting company's spam filtering?

We have DSL with a dynamic IP. I was thinking that's another reason why we shouldn't bother using the SBS box for hosting the mail directly?

Who is Participating?

It is correct in that the native pop3 connector in SBS 2003 does not support the IMF.
The pop3 connector delivers mail directly to the exchange store, while the smtp connector delivers it through event sinks. One of these event sinks is the IMF, so to sum up you can only use the IMF with the SMTP connecter.
But you can use a thrird party pop3 to smtp converter, which will connect to your pop3 addresses and then deliver the mail directly to the smtp connector, but in general most of these are not free.
For a list of third party products check out www.slipstick.com theyy are a great resource for Exchange & Outlook information, updates, plugins and third party tools.

If you could get a static ip address from your serice provider on a package that does not block port 25, you could host your own smtp.
If you were to go this route you could still continue to use your isps sharthost / smtp server, or setup an A recorn and an MX record and a reverse DNS record for your static IP address, but again these things cost money.

Another alternative again is to ask you ISP to provide a Backup SMTP server that your exchange server will request emails from. All your emails would go to the isps smtp server and exchange would query it for any new messages. Again this will cost.